U.S. Pat. No. 4,652,015 entitled "Security Paper for Currency and Bank Notes" describes a security device in the form of a metallized plastic thread that is incorporated within a security paper such as bank notes and other valuable documents during the papermaking process. The security thread is virtually invisible under reflected light yet readily discernible under transmitted light.
U.S. Pat. No. 4,980,569 entitled "Security Paper Verification Device" describes the combination of photo diodes and photo transistors positioned on the opposing sides of the currency to optically ascertain the presence of the security thread within the currency and to determine whether or not the security thread is on the surface of the currency.
Other currency verification devices, such as used with vending machines and the like, detect the presence of iron oxide within the ink that is printed on the face side of U.S. currency to verify the authenticity of the preferred currency. Currency of lower denomination bills can be bleached and photoprinted to a larger denomination using available color photocopy equipment. Since the position of the metallized thread in modern U.S. currency corresponds to the currency denomination and the thread is denominated, such earlier counterfeiting schemes are no longer workable.
Metal detection apparatus in the form of proximity detectors using capacitive circuits and magnetic detectors using tuned resonance circuits are currently employed to rapidly determine the presence of both ferrous and non-ferrous metals for a variety of applications. It is believed that such metal detection circuits in combination with the optical circuits described within aforementioned U.S. Pat. No. 4,980,569 could provide effective and inexpensive means for currency verification in supermarkets, banks and the like.